Diffusion in ordinary and supercooled liquids

Summary We discuss a simplified approach for the recollision part of the memory function for self-motion in ordinary and supercooled liquids. This part accounts for the substantial decrease of the diffusion coefficient with respect to the binary prediction, an effect which becomes larger and larger on cooling the system. The quantitative results compare satisfactorily with previous and new simulation data obtained for a liquid rubidium model both near the triple point and in moderately supercooled states. Paper presented at the workshop ?Highlights on Simple Liquids”, held in Turin at ISI on 1–3 May, 1989.     

Heat and mass transfer at adiabatic evaporation of binary zeotropic solutions

Results of numerical simulation of heat and mass transfer in a laminar flow of three-component gas at adiabatic evaporation of binary solutions from a flat plate are presented. The studies were carried out for the perfect solution of ethanol/methanol and zeotrope solutions of water/acetone, benzene/acetone, and ethanol/acetone. The liquid-vapor equilibrium is described by the Raoult law for the ideal solution and Carlson–Colburn model for real solutions. The effect of gas temperature and liquid composition on the heat and diffusion flows, and temperature of vapor-gas mixture at the interface is analyzed. The formula for calculating the temperature of the evaporation surface for the binary liquid mixtures using the similarity of heat and mass transfer was proposed. Data of numerical simulations are in a good agreement with the results of calculations based on the proposed dependence for all examined liquid mixtures in the considered range of temperatures and pressures.     

Exact Calculation of Antiferromagnetic Ising Model on an Inhomogeneous Surface Recursive Lattice to Investigate Thermodynamics and Glass Transition on Surface/Thin Film

An inhomogeneous 2-dimensional recursive lattice formed by planar elements has been designed to investigate the thermodynamics of Ising spin system on the surface/thin film. The lattice is constructed as a hybrid of partial Husimi square lattice representing the bulk and 1D single bonds representing the surface. Exact calculations can be achieved with the recursive property of the lattice. The model has an anti-ferromagnetic interaction to give rise to an ordered phase identified as crystal, and a solution with higher energy to represent the amorphous/metastable phase.Free energy and entropy of the ideal crystal and supercooled liquid state of the model on the surface are calculated by the partial partition function. By analyzing the free energies and entropies of the crystal and supercooled liquid state,we are able to identify the melting and ideal glass transition on the surface. The results show that due to the variation of coordination number, the transition temperatures on the surface decrease significantly compared to the bulk system.Our calculation qualitatively agrees with both experimental and simulation works on the thermodynamics of surfaces and thin films conducted by others. Interactions between particles farther than the nearest neighbor distance are taken into consideration, and their effects are investigated.     

On the potential energy landscape of supercooled liquids and glasses

The activation-relaxation technique (ART), a saddle-point search method, is applied to determine the potential energy landscape around supercooled and glassy configurations of a three-dimensional binary Lennard-Jones system. We show a strong relation between the distribution of activation energies around a given glassy configuration and its history, in particular, the cooling rate used to produce the glass and whether or not the glass was plastically deformed prior to sampling. We also compare the thermally activated transitions found by ART around a supercooled configuration with the succession of transitions undergone by the same supercooled liquid during a time trajectory simulated by molecular dynamics. We find that ART is biased towards more heterogeneous transitions with higher activation energies and more broken bonds than the MD simulation.     

Mechanical responses and stress fluctuations of a supercooled liquid in a sheared non-equilibrium state

A steady shear flow can drive supercooled liquids into a non-equilibrium state. Using molecular dynamics simulations under steady shear flow superimposed with oscillatory shear strain for a probe, non-equilibrium mechanical responses are studied for a model supercooled liquid composed of binary soft spheres. We found that even in the strongly sheared situation, the supercooled liquid exhibits surprisingly isotropic responses to oscillating shear strains applied in three different components of the strain tensor. Based on this isotropic feature, we successfully constructed a simple two-mode Maxwell model that can capture the key features of the storage and loss moduli, even for highly non-equilibrium state. Furthermore, we examined the correlation functions of the shear stress fluctuations, which also exhibit isotropic relaxation behaviors in the sheared non-equilibrium situation. In contrast to the isotropic features, the supercooled liquid additionally demonstrates anisotropies in both its responses and its correlations to the shear stress fluctuations. Using the constitutive equation (a two-mode Maxwell model), we demonstrated that the anisotropic responses are caused by the coupling between the oscillating strain and the driving shear flow. Due to these anisotropic responses and fluctuations, the violation of the fluctuation-dissipation theorem (FDT) is distinct for different components. We measured the magnitude of this violation in terms of the effective temperature. It was demonstrated that the effective temperature is notably different between different components, which indicates that a simple scalar mapping, such as the concept of an effective temperature, oversimplifies the true nature of supercooled liquids under shear flow. An understanding of the mechanism of isotropies and anisotropies in the responses and fluctuations will lead to a better appreciation of these violations of the FDT, as well as certain consequent modifications to the concept of an effective temperature.     

Pressure and Time Dependences of the Supercooled Liquid-to-Liquid Transition in Sulfur

Thermal behavior of bulk amorphous sulfur is investigated by in situ temperature measurements at high pressures of 0.9,1.4 and 2.1 GPa,and under different heating rates of 8,10 and 12K/min at 0.9 GPa.The results show that the onset temperature of the transition from the supercooled liquid to the liquid state for sulfur increases with the pressure and the heating rate.It is deduced that the transition does not follow the Clapeyron equation,indicating considerable coupling of the molecular structure change in the transition.Along with the data at ambient pressure and high pressure,we present a dynamic diagram to demonstrate the relationship between the amorphous solid,supercooled liquid,liquid,and crystal phases of sulfur,and suggest an experimental approach to establish pressure-temperature-time transition diagrams for supercooled liquid and liquid.     

Phase separation in nematic and smectic a phases

Until recently no example was known with a miscibility gap separating two non-ordered mesophases (i.e.: nematic, smectic A or C) of the same symmetry. In most cases of binary solutions it was observed that either the phase diagram is of the simple eutectic type with quasi ideal behaviour of the mixtures or the liquid crystalline phases are destabilized at the benefit of the isotropic solution (Figure 1). These properties of easy miscibility among mesogens have been long and widely applied at different stages of the study and application of liquid crystals.     

Surface and interface effects on structural transformation of vapor-deposited ethylbenzene films

We have investigated how the structures of vapor-deposited glassy films change with increasing temperature by using time-of-flight secondary ion mass spectrometry and ion scattering spectroscopy. It is found that intermixing of the topmost layer of an ethylbenzene film occur at temperature (~ 80 K) considerably lower than the glass transition temperature (T_g = 118 K) when the film is deposited at 20 K. This phenomenon can be interpreted as the occurrence of a two-dimensional liquid that diffuses into pores of the film, which is evidenced from comparison with surface diffusivity measurements using a porous silicon layer. For nonporous films deposited at higher temperatures, the molecules intermix gradually prior to the abrupt film morphology change at T_g. This phenomenon can be interpreted as decoupling between translational diffusivity and viscosity in the bulk. The film thickness has no significant effects on the evolution of supercooled liquid at T_g except for the monolayer film, whereas crystallization is quenched for the films thinner than 8 monolayers. The roles of the 2D liquid on the surface and an immobilized layer formed at the interface are discussed in finite-size effects on the glass-liquid transition and crystallization.     

Estimation of the intrinsic heterogeneity of ionic glass-forming melts

The supercooled ionic melt is considered as a kind of binary composite material with an intrinsic heterogeneity (liquid medium and denser packed clusters) dependent on temperature. The conductivities of phases are extracted from the high (liquid) and low (glass) temperature experimental data. Effective conductivity of such a composite has been estimated using the checkerboardlike model with the presence of heterogeneity on different length scales. Using this model the volume fraction of the denser inclusions from experimental data on Ca2K3(NO3)(5) and Zr-Ba-La-Al-Na-F compound glass formers has been estimated.     

Solid/liquid interfacial free energies in binary systems

The interfacial segregation and the free energy of segregation for solid/liquid interfaces between binary solutions are computed for the (111) boundary of face-centered-cubic crystals. A lattice-liquid interfacial model and pair-bonded regular solution model are employed in the treatment with an accommodation for liquid interfacial entropy. It is concluded that the zone of compositional transition across the interface is generally a few atomic layers in width and is moderately narrower for ideal solutions. The free energy of the segregated interface depends primarily upon the solid composition and the heats of fusion of the component atoms, the composition difference of the solutions, and the difference of the heats of mixing of the solutions. Master plots are presented for predicting the segregation and interfacial free energies in general binary systems.     

A simple capacitive cell for the measurement of liquids dielectric constant under transient thermal conditions

A simple device for the measurement of the complex dielectric permittivity of liquids in various thermodynamic states has been developed. It uses a cylindrical aluminium capacitor of a type currently applied in tuning antenna circuits. The capacitor is filled with the liquid solution under study. A comparison of its capacity is made with that of the nitrogen filled capacitor tested under the same thermal conditions. This comparison allows the determination of the real and imaginary part of the solutions permittivity as a function of temperature (between 150 and 300 K) and frequency (between 100 Hz to 2 MHz). After validating the technique with pure glycerol and pure 1,2-propanediol, spectroscopic measurements have been undertaken on pure and diluted 1,2-propanediol in water. Due to the low heat capacity and the high thermal conductivity of the capacitor, cooling rates of 40 K/min have been achieved inside the solution, allowing measurements in the supercooled liquid and vitreous states. Results are presented and discussed in terms of relaxation and the physical states of the sample. By selecting the required thermal conditions, this device permits the observation of thermal transitions, such as ice crystallisation, and measurements to be conducted in the unstable supercooled liquid state. These measurements are necessary in the development of an effective electromagnetic warming device for vitrified cryoprotective solutions.     

Nonlinear fluctuating hydrodynamics and sequence of time scales of relaxation in supercooled liquids

Nonlinear fluctuating hydrodynamic (NFH) models for relaxation in the supercooled liquid are considered. Recent results on self consistent mode coupling theory for the slow relaxation of density fluctuations are analyzed to explain the glassy dynamics. The relaxation mechanisms for different types of models with and without wave vector dependences are discussed. For the schematic models where all wave vector dependences are dropped a sequence of time scales enters the relaxation process. For the non-ergodicity parameter very close to the ideal transition point is scaled by an exponent equal to 1/2. This is demonstrated here through an analysis of the mode-coupling equations for the wave vector dependent models that follow from equations of NFH.     

Dynamics of supercooled confined water measured by deep inelastic neutron scattering

In this paper, we present the results of deep inelastic neutron scattering (DINS) measurements on supercooled water confined within the pores (average pore diameter ~ 20 Å) of a disordered hydrophilic silica matrix obtained through hydrolysis and polycondensation of the alkoxide precursor Tetra-Methyl-Ortho-Silicate via the sol-gel method. Experiments were performed at two temperatures (250 K and 210 K, i.e., before and after the putative liquid–liquid transition of supercooled confined water) on a “wet” sample with hydration h ~ 40% w/w, which is high enough to have water-filled pores but low enough to avoid water crystallization. A virtually “dry” sample at h ~ 7% was also investigated to measure the contribution of the silica matrix to the neutron scattering signal. As is well known, DINS measurements allow the determination of the mean kinetic energy and the momentum distribution of the hydrogen atoms in the system and therefore, allow researchers to probe the local structure of supercooled confined water. The main result obtained is that at 210 K the hydrogen mean kinetic energy is equal or even slightly higher than at 250 K. This is at odds with the predictions of a semiempirical harmonic model recently proposed to describe the temperature dependence of the kinetic energy of hydrogen in water. This is a new and very interesting result, which suggests that at 210 K, the water hydrogens experience a stiffer intermolecular potential than at 250 K. This is in agreement with the liquid–liquid transition hypothesis.     

Scaling behavior in the dynamics of a supercooled Lennard-Jones mixture

Summary We present the results of a large-scale molecular-dynamics computer simulation of a binary, supercooled Lennard-Jones fluid. At low temperatures and intermediate times the time dependence of the intermediate scattering function is well described by a von Schweidler law. The von Schweidler exponent is independent of temperature and depends only weakly on the type of correlator. For long times the correlation functions show a Kohlrausch behavior with an exponent β that is independent of temperature. This dynamical behavior is in accordance with the mode-coupling theory of supercooled liquids. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Measurements of fractal dimensionality in 0.1Cs2O-0.9TeO2 glass-forming tellurite system by Raman spectroscopy

Low-frequency Raman spectroscopy was utilized in order to estimate the fractal dimensionality for tellurite 0.1Cs₂O-0.9TeO₂ binary glass-forming system in a temperature range, which includes the glassy and supercooled liquid state. A variation in fractal dimensionality was observed and calculated directly from the Raman spectroscopic data. This variation can be interpreted as an indication of structural alterations caused by temperature variation and correlated with existing structural models concerning tellurite network. Low-frequency Raman spectroscopy has proved to be a valuable tool for studying the fractal dimensionality and structural transformation in amorphous phases. The estimation of the localization degree of lattice vibration in disordered materials is important and discussed in the context of current phenomenological status of the field.     

Democratic particle motion for metabasin transitions in simple glass formers

We use molecular dynamics computer simulations to investigate the local motion of the particles in a supercooled binary liquid. Using the concept of the distance matrix, we find that the alpha relaxation corresponds to a small number of crossings from one metabasin to a neighboring one. Each crossing is very rapid and involves the collective motion of O(40) particles that form a relatively compact cluster, whereas stringlike motions seem not to be relevant for these transitions. These compact clusters are thus potential candidates for the cooperatively rearranging regions proposed a long time ago by Adam and Gibbs.     

The structure of ambient water

We review the spectroscopic techniques and scattering experiments used to probe the structure of water, and their interpretation using empirical and ab initio models, over the last 5 years. We show that all available scientific evidence overwhelmingly favors the view of classifying water near ambient conditions as a uniform, continuous tetrahedral liquid. While there are controversial issues in our understanding of water in the supercooled state, in confinement, at interfaces, or in solution, there is no real controversy in what is understood as regards bulk liquid water under ambient conditions.     

Observation and analysis of a glass transition in amorphous frozen solutions of iron-II chloride

This paper deals with Mössbauer investigations, X-ray diffraction studies and differential calorimetric measurements of the amorphous state of frozen solutions of FeCl₂ in water. This glassy state persists from at least ?180 °C until ?90 °C. All three experimental methods reveal the existence of a glass transition at ?110 °C from an amorphous state to a supercooled liquid. It is shown that for such transitions important conclusions can be drawn from a comparison between the Mössbauer and X-ray diffraction Debye-Waller factor respectively by determining the transmitted Mössbauer intensity far off resonance. Out of the analysis we conclude that the glassy state of quenched ice is due to the hexaquo complexes as implanted impurities which prevail their surrounding from a regular crystallisation. These impurities are also responsible for the glass transition into a supercooled liquid state by releasing new degrees of freedom as e.g. hindered rotational modes.     

过冷液体钾形核初期微观动力学机理的模拟研究

采用分子动力学方法对液态金属钾凝固过程进行了模拟,根据凝固过程体系平均原子能量、原子成键类型和成团类型,以及均方位移和非Gauss参数等动力学参数的演化特征,对过冷熔体形核初期微观动力学机理进行了研究.结果表明:根据过冷液体钾结晶形核过程热力学、动力学和结构特性的演化规律, 其过冷温度区间可以分为两个明显不同的阶段,潜在结晶核心出现在过冷液体较低温区.过冷熔体钾在形核初期,二十面体团簇结构在α-弛豫阶段逐渐解体,同时具有体心立方(bcc)结构的潜在结晶核心逐步形成,其临界晶核包含约300个原子.